The U.S. State Department's third-ranking official, Undersecretary of State for Political Affairs Tom Shannon, is stepping down from his post, the State Department said on February 1.

Shannon, who is the latest senior diplomat to exit since President Donald Trump took office a year ago, said in a letter to the department staff that he is retiring for personal reasons.

"My decision is personal and driven by a desire to attend to my family, take stock of my life, and set a new direction for my remaining years," Shannon wrote.

State Department spokeswoman Heather Nauert said Shannon, 60, agreed to Secretary of State Rex Tillerson's request that he stay on until his successor is named.

Nauert said Shannon had served under six presidents and 10 secretaries of state.

Shannon is the top career U.S. diplomat and a nearly 35-year veteran of the U.S. Foreign Service. He was the most senior department official to remain in his job after the transition from President Barack Obama's administration to that of President Donald Trump.

Trump has to appoint a replacement for Shannon, who would need Senate confirmation.

Shannon's departure deprives the State Department of a seasoned veteran at a time of growing diplomatic concerns including the threat posed by a nuclear-armed North Korea.

Tillerson has faced criticism over his leadership of the State Department, with some officials being quoted as saying he has weakened U.S. diplomacy by removing or pushing out senior career officials while failing to nominate or to win Senate confirmation for officials to fill key agency roles.

Based on reporting by AP and Reuters